Turbochargers are devices that are frequently used to increase the output of an internal combustion engine. A typical turbocharger comprises a turbine wheel coupled to a compressor impeller by a common shaft. Exhaust gas from the engine is diverted into a turbine housing of the turbocharger and through an inlet nozzle. The exhaust gas is directed onto the turbine wheel, causing it to spin, which in turn spins the common shaft and the compressor impeller.
The compressor impeller is disposed within a compressor housing having an air inlet and a pressurized or boosted air outlet. The spinning compressor impeller operates to pressurize air entering the compressor housing and generate a pressurized or boosted air stream that is directed into an inlet system of the internal combustion engine. This boosted air is mixed with fuel to provide a combustible mixture within the combustion chambers of an engine. In this manner, the turbocharger operates to provide a larger air mass and fuel mixture, than otherwise provided via an ambient pressure air intake stream, that results in a greater engine output during combustion. The gain in engine output that can be achieved is directly proportional to the increase in intake air flow pressure generated by the turbocharger. However, allowing the boost pressure to reach too high a level can result in severe damage to both the turbocharger and the engine, particularly when the engine has to operate beyond its intended performance range.
Thus, an objective of turbocharger design is to regulate or control the boost pressure provided by the turbocharger in a manner that optimizes engine power output at different engine operating conditions without causing engine damage. A known technique for regulating boost pressure is by using a turbocharger having a variable geometry member that functions to control the amount of exhaust gas directed to the turbine wheel. Turbochargers comprising such variable geometry members are referred to as variable geometry turbochargers (VGTs).
One type of VGT includes a variable geometry member in the form of multiple adjustable-position vanes that are positioned within the turbine housing, and that are movable within in inlet nozzle of the turbine housing to regulate the amount of exhaust gas that is passed to the turbine wheel. The vanes in this type of VGT can be opened to permit greater gas flow across the turbine wheel, causing the turbine wheel to spin at a higher speed and raise the boost pressure, or closed to restrict exhaust gas flow to the turbine, thereby reducing the boost pressure. Thus, the amount of boost pressure generated by this type of VGT can be regulated by varying the vane position so as to optimize engine output while avoiding engine damage.
Control systems for such VGTs are known in the art, and typically involve a closed-loop control methodology that involves an iterative process of monitoring a number of engine and turbocharger operating parameters, and providing a control output based on such inputs. For example, such control system may include a number of sensors to measure such parameters as actual boost pressure, fuel flow, ambient air pressure, engine speed, and the like, and may involve using a boost map for the purpose of comparing the actually-measured boost pressure to desired boost pressures at particular engine operating conditions. Using these parameters and/or stored data, such control systems operate to adjust the vane position to regulate the flow of exhaust across the turbine wheel to match the actual boost pressure to the desired boost pressure.
Although effective, conventional VGT control systems tend to be complicated and expensive to implement, for example, based on the relatively large number of sensors needed to monitor the desired operating parameters, and based on the significant number of calculations that must be performed to achieve the desired control output. Additionally, because such conventional systems are based on a closed-loop control methodology, certain stability issues and challenges are known to exist.
It is, therefore, desired that a control system for use with VGTs be designed and constructed in a manner that can provide relatively cost effective, efficient, and simple control of turbocharger operation. It is also desired that such control system provide reliable position control, of variable geometry members within such VGT, regardless of variabilities that can exist on the variable geometry members, e.g., such as external forces or hydraulic pressures.